In general, when a work is cut using a cutting tool such as a wire saw, a band saw or the like, a cutting liquid has widely been used as a lubricant between the cutting tool and the work, to remove a friction heat therebetween and clean cutting chips away from the cutting tool and the work. For example, when wafers are produced by slicing an ingot of silicon single crystal as a work, a non-aqueous cutting liquid in a slurry state, which is prepared by dispersing loose abrasive grains of SiC (silicon carbide) or the like in a dispersion medium composed of a mineral oil and higher hydrocarbons as major components, has widely been used.
A wafer obtained by slicing has been cleaned with a low-priced chlorinated organic solvent with high detergency such as trichloroethane and dichloromethane.
In this way, in a system in which cutting is performed by dynamic contact among a cutting tool, a work and loose abrasive grains, to increase dispersibility of loose abrasive grains in a cutting liquid is important in order to keep the cutting ability at a constant level all time. As methods of increasing the dispersibility, there have been available two methods in a broad sense: (a) a method in which a dispersing agent is added in a dispersion medium and (b) a method in which a thickener is added in a dispersion medium.
The method (a) is to increase the dispersibility of abrasive grains themselves positively. Generally dispersibility of particles in a fluid is increased and a sedimentation speed of the particle is decreased, when individual particles each have a small mass, a sufficient repulsive force works between particles due to factors such as an electric double layer, or steric hindrance by absorbed molecules on a particle surface or the like and thereby the particles are existent as primary particles (particles in a non-aggregation state). Hence, an electrolyte or a surfactant having a lipophilic group such as an alkyl chain with a sufficient length has been added as a dispersing agent.
On the other hand, the method (b) is to increase a viscosity of a dispersion medium and thereby hinder Brownian movement of an abrasive grain in order to diminish a sedimentation speed. As a thickener, there has been known bentonite.
However, there are many problems to be solved for a nonaqueous cutting liquid.
First of all, organic solvents which have widely been known as a dispersion medium for a conventional nonaqueous cutting liquid are strong in smell and have inflammability according to a kind thereof. Hence the organic solvents have causes to deteriorate a working environment.
An intermediate product which has been obtained by cutting with a cutting oil, as described above, requires cleaning with an organic solvent capable of eliminating residue of the cutting oil on the product. However, since dichloroethane, for example, which has well been used in cleaning a semiconductor wafer is designated as a material to deplete the ozone layer by the Government, a usage quantity of the compound has to be decreased toward the perfect disuse state hereinafter, but economy and cleaning ability of alternatives are still short of target levels in the current state.
The method (a) in which a dispersing agent is added has a problem that precipitate is formed as a hard cake.
Abrasive grains which are increased in dispersibility by addition of a dispersing agent have, for certain, a slow sedimentation speed but are compressed under a load which exceeds a repulsive force while being put into close contact to each other in the course of sedimentation to form a hard cake. Once a hard cake is formed, it is hard to be again dispersed into a state same as the original one. Accordingly, if such a cutting liquid which has produced precipitate is tried to recycle after stirring for a long time, the cutting liquid in reuse as a result is in a state of a low concentration of abrasive grains, which decreases cutting ability. Besides, there also arise other problems that piping of a supply system of the cutting liquid is clogged, a tool for pulverization of a hard cake is worn at an earlier stage or the like.
If the mass of each of abrasive grains is large and a repulsive force therebetween is small, a problem of formation of a hard cake is eliminated. The reason why is that abrasive grains electrically bond each other with multivalent ions, which are mainly existent in the dispersion medium, interposed therebetween and as a result, multipored, soft flocculates are formed, so that relatively soft precipitate (soft cake) is formed over time. If the precipitate is of a soft cake, redispersion is easy to be effected.
However, since abrasive grains in a dispersion medium are hard to be kept in the primary particle state and in addition, a sedimentation speed of flocculates in the dispersion medium is fast, a distribution of abrasive grain concentration in the cutting liquid is apt to be uneven and thus cutting ability is easy to be unstable.
For this reason, a cutting liquid with high dispersibility is eventually forced to be employed and a cutting liquid which is hard to be recycled is collected after the use and generally incinerated for disposal. In this incineration, much of carbon dioxide caused by combustion of an organic solvent is released, which is not preferable from the viewpoint of prevention of global warming.
On the other hand, the method (b) in which a thickener is added has a guaranteed effect, on the assumption that a viscosity of a cutting liquid is unchanged, whereas the viscosity of a cutting liquid is actually changed due to a variety of factors.
A viscosity of a cutting liquid is generally increased if cutting chips are mixed into the liquid. Since abrasive grains cannot be supplied at a constant rate on a cutting surface of a work in a uniform manner as a viscosity increases, it is necessary for the cutting liquid to be replaced with a new one when a mixed amount of cutting chips is accumulated to 3 to 4% by weight of the total of the cutting liquid. This replacement increases a waste amount of the cutting liquid, which in turn increases an amount of carbon dioxide produced by incineration of the cutting liquid waste.
Viscosity of a nonaqueous cutting liquid is also increased as water is mixed into the liquid. Therefore, in order to prevent water from mixing, there has been a tight restriction imposed on a cleaning operation of wafers and a mounting base for ingots in a wire saw machine. That is, since a cleaning liquid has to be an organic solvent in a system in which a nonaqueous cutting liquid is employed, there is a requirement that cleaning is operated with an independent tank filled with a cleaning liquid, different from a cutting liquid tank. Hence, an installment area for cutting facilities is increased and besides, the usage amount of an organic solvent is also increased, which causes not only a working environment but a global environment to be further deteriorated.
To the contrary, when a molecular structure of a dispersing agent is broken through disconnection of an atomic bond in a molecule, viscosity of the nonaqueous cutting liquid is decreased and a cutting ability becomes unstable.
As described above, when a nonaqueous cutting liquid is employed, it is very difficult to establish compatibility between achievement of high dispersibility of abrasive grains which is required for maintenance of a cutting ability and prevention of a hard cake from forming which is required for improvement on recyclability of the cutting liquid and maintainability of the facilities, while suppressing an impact on a working environment and a global environment.
Therefore, the present invention, in order to solve the problems, has an object to propose a new aqueous composition which constitutes a base for an aqueous cutting liquid and provide an aqueous cutting liquid using the composition, producing processes for the composition and the cutting liquid and a cutting method using the cutting liquid.